# Simple Setup for Measuring the Response to Differential Mode Noise of Common Mode Chokes

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## Abstract

**:**

## 1. Introduction

## 2. Analysis

#### 2.1. Modal Analysis of the CMC

#### 2.2. Effect of Electric Coupling to Metallic Surfaces

#### 2.3. Effect of Magnetic Coupling to Metallic Surfaces

## 3. Results

#### 3.1. Analysis of the Response of a Standalone CMC

#### 3.2. Effect of Capacitive Couplings in a PCB

#### 3.3. Effect of Magnetic Coupling to Nearby Conducting Surfaces

## 4. Discussion and Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Representation and circuit model of a common mode choke made up of two equal coupled windings with self-inductance L and mutual inductance M.

**Figure 2.**Equivalent circuits of the modes obtained for the high-frequency circuit of the CMC in Figure 1b. Normalized voltages at the four terminals of the CMC are indicated for each mode.

**Figure 3.**Balanced and unbalanced setups for measuring transmission coefficients conveying information about the attenuation provided by a CMC for a differential mode excitation. Measurements can be performed with a spectrum analyzer (SA) with tracking generator (TG) or a VNA.

**Figure 4.**Schematic of a CMC mounted on a grounded PCB connected in UDM setup. Transmission lines labelled as ${\mathrm{TL}}_{\mathrm{C}}$ stand for the interconnecting cables. Transmission lines labelled as ${\mathrm{TL}}_{\mathrm{T}}$ represent the signal traces of the PCB. The signal traces terminated as open-circuits at terminals 2 and 4 are supposed to be electrically short and thereby modeled as two capacitances ${C}_{r}$ to ground.

**Figure 5.**Equivalent circuit of the UDM setup for a CMC mounted on a PCB with an ungrounded return plane. Transmission lines labelled as ${\mathrm{TL}}_{\mathrm{C}}$ stand for the interconnecting cables. Transmission lines labelled as ${\mathrm{TL}}_{\mathrm{T}}$ represent the signal traces of the PCB. Since all the signal traces are terminated as open-circuits and they are supposed to be electrically short, they are actually modeled as capacitances ${C}_{r}$ to ground.

**Figure 6.**Star to triangle conversion for the parasitic capacitances ${C}_{r}$ that appear at the four terminals of the CMC in the circuit of Figure 5.

**Figure 7.**Open-circuit (OC) setup proposed in [16] for characterizing CMCs.

**Figure 10.**A 2.2mH CMC, listed in Table 2 as WÜRTH ELEKTRONIK 744824622, mounted on a PCB fabricated to check the impact on $|{S}_{21}^{\mathrm{UDM}}|$ of the capacitive coupling of the signal traces to the return plane.

**Figure 11.**Measured and calculated $|{S}_{21}^{\mathrm{UDM}}|$ curves for the CMC listed as WÜRTH ELEKTRONIK 744824220 (20 mH) in Table 2. We compare curves for three cases: standalone CMC, CMC mounted on a PCB with a floating return plane and CMC mounted on a PCB whose return plane is grounded (G label).

**Figure 12.**Measured and calculated $|{S}_{21}^{\mathrm{DM}}|$ (

**a**) and $|{S}_{21}^{\mathrm{UDM}}|$ (

**b**) for the CMC listed as WÜRTH ELEKTRONIK 744824310 (10 mH) in Table 2, whose picture is inserted in the figures. The CS acronym used in the legends indicates results corresponding to the case where a conducting surface is placed near the CMC.

**Table 1.**Transmission coefficients and frequencies of resonance for a CMC measured in the setups of Figure 3, where ${Y}_{\mathrm{OC}}={Y}_{\mathrm{H}}{Y}_{\mathrm{D}}/({Y}_{\mathrm{H}}+{Y}_{\mathrm{D}})$. Approximated expressions assume ${C}_{g}\ll {C}_{t},{C}_{w}$ and ${Y}_{\mathrm{C}}\ll {Y}_{\mathrm{V}},{Y}_{\mathrm{H}},{Y}_{\mathrm{D}}$.

Setup | Transmission Coefficient | Frequencies of Resonance |
---|---|---|

DM | ${S}_{21}^{\mathrm{DM}}=\frac{R{Y}_{\mathrm{D}}}{2+R{Y}_{\mathrm{D}}}-\frac{R{Y}_{\mathrm{V}}}{2+R{Y}_{\mathrm{V}}}$ | ${f}_{\mathrm{DM}}=\frac{1/2\pi}{\sqrt{{C}_{t}{L}_{\mathrm{DM}}}}$ |

UDM | ${S}_{21}^{\mathrm{UDM}}\approx \frac{R({Y}_{\mathrm{D}}+{Y}_{\mathrm{V}})}{2+R({Y}_{\mathrm{D}}+{Y}_{\mathrm{V}})}$ | ${f}_{\mathrm{UDM}}\approx \frac{1/2\pi}{\sqrt{({C}_{t}+2{C}_{w}){L}_{\mathrm{DM}}}}$ |

OC | ${S}_{21}^{\mathrm{OC}}\approx \frac{2R{Y}_{\mathrm{OC}}}{2R{Y}_{\mathrm{OC}}+1}$ | ${f}_{\mathrm{OC}}\approx \frac{1/2\pi}{\sqrt{({C}_{t}+{C}_{w}){L}_{\mathrm{DM}}}}$ |

**Table 2.**Parameters extracted for the equivalent circuit of Figure 1b for several commercial common mode chokes.

Manufacturer and Part Number | L (mH) | ${\mathit{L}}_{\mathbf{CM}}\phantom{\rule{0.166667em}{0ex}}$ (mH) | ${\mathit{L}}_{\mathbf{DM}}\phantom{\rule{0.166667em}{0ex}}$ (uH) | ${\mathit{C}}_{\mathit{w}}\phantom{\rule{0.166667em}{0ex}}$ (pF) | ${\mathit{C}}_{\mathit{t}}\phantom{\rule{0.166667em}{0ex}}$ (pF) | ${\mathit{R}}_{\mathbf{CM}}$ (k$\mathbf{\Omega}$) | ${\mathit{R}}_{\mathbf{DM}}$ (k$\mathbf{\Omega}$) |
---|---|---|---|---|---|---|---|

WÜRTH ELEKT. 744824622 | 2.2 | 4.94 | 4.7 | 4.2 | 6.8 | 17.2 | 6.5 |

WÜRTH ELEKT. 744824310 | 10 | 26.7 | 33.6 | 4.7 | 18.3 | 118 | 16.6 |

WÜRTH ELEKT. 744824220 | 20 | 54.1 | 57.6 | 10.7 | 20.2 | 203 | 22.2 |

WÜRTH ELEKT. 7448011008 | 8.0 | 6.90 | 6.5 | 0.86 | 2.7 | 22.1 | 8.1 |

MURATA PLA10AN2230R4D2B | 22 | 71.3 | 173 | 1.8 | 2.9 | 73.9 | 33.0 |

KEMET SC-02-30G | 3.0 | 7.40 | 5.8 | 1.4 | 2.8 | 34.3 | 16.7 |

KEMET SCF20-05-1100 | 11 | 13.4 | 5.1 | 8.6 | 7.2 | 15.4 | 7.9 |

**Table 3.**Measured frequencies of resonance of ${S}_{21}^{\mathrm{UDM}}$ for three CMCs when isolated and when mounted on a PCB with an ungrounded return plane. Also, calculated ${f}_{\mathrm{UDM}}$ for the latter case.

${\mathit{f}}_{\mathbf{UDM}}$ (MHz) | |||
---|---|---|---|

CMC Isolated | CMC on Ungrounded PCB | ||

CMC Part Number | Measured | Measured | Calculated |

WE 744824622 | 16.9 | 13.6 | 12.8 |

WE 744824310 | 4.77 | 4.30 | 4.07 |

WE 744824220 | 3.02 | 2.65 | 2.74 |

**Table 4.**Measured frequencies of resonance of ${S}_{21}^{\mathrm{UDM}}$ for three CMCs when isolated and when mounted on a PCB with a grounded return plane. Also, calculated ${f}_{\mathrm{UDM}}$ for the latter case.

${\mathit{f}}_{\mathbf{UDM}}$ (MHz) | |||
---|---|---|---|

CMC Isolated | CMC on Grounded PCB | ||

CMC Part Number | Measured | Measured | Calculated |

WE 744824622 | 16.9 | 21.7 | 22.8 |

WE 744824310 | 4.77 | 5.63 | 5.64 |

WE 744824220 | 3.02 | 3.40 | 3.72 |

**Table 5.**${L}_{\mathrm{DM}}$ for different CMCs with and without the effect of a nearby conducting surface.

CMC Part Number | ${\mathit{L}}_{\mathbf{DM}}$ ($\mathsf{\mu}$H) | ${\mathit{L}}_{\mathbf{DM}}^{\mathbf{CS}}$ ($\mathsf{\mu}$H) |
---|---|---|

WE 744824622 | 4.73 | 3.38 |

WE 744824310 | 33.6 | 27.0 |

WE 744824220 | 58.2 | 44.2 |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

González-Vizuete, P.; Domínguez-Palacios, C.; Bernal-Méndez, J.; Martín-Prats, M.A. Simple Setup for Measuring the Response to Differential Mode Noise of Common Mode Chokes. *Electronics* **2020**, *9*, 381.
https://doi.org/10.3390/electronics9030381

**AMA Style**

González-Vizuete P, Domínguez-Palacios C, Bernal-Méndez J, Martín-Prats MA. Simple Setup for Measuring the Response to Differential Mode Noise of Common Mode Chokes. *Electronics*. 2020; 9(3):381.
https://doi.org/10.3390/electronics9030381

**Chicago/Turabian Style**

González-Vizuete, Pablo, Carlos Domínguez-Palacios, Joaquín Bernal-Méndez, and María A. Martín-Prats. 2020. "Simple Setup for Measuring the Response to Differential Mode Noise of Common Mode Chokes" *Electronics* 9, no. 3: 381.
https://doi.org/10.3390/electronics9030381